The limited availability of and environmental concerns about fossil fuels make them increasingly less attractive as a means to produce electricity. As a result of this trend, alternative energy sources, particularly solar energy, are becoming more popular. While solar energy is a reliable and dependable energy source, the costs associated with solar energy production have traditionally limited its availability and desirability as a substitute for fossil fuels. However, recent technological advances in solar cell manufacturing show promise to lower the cost of solar energy.
Solar energy proponents and researchers state that higher solar cell efficiency and lower production costs are two ways to reduce the overall cost of solar energy. In particular, solar cells with absorber materials comprised of copper, indium, gallium, and selenium and/or sulfur [hereinafter Cu(In,Ga)(S,Se)2 or CIGS] show promise in higher efficiency, lower production costs, and long operational lifetimes. These absorber materials are the result of innovative thin-film manufacturing technologies that further reduce manufacturing costs by lowering raw material costs and increasing throughput and efficiencies.
As is commonly practiced in the art, these CIGS cells are manufactured in either a one-stage thermal co-evaporation process or a two-stage process. The single stage thermal co-evaporation process consists of depositing all of the CIGS elements onto a substrate and simultaneously heating that substrate temperature to approximately 450° C. to 600° C. to allow the constituent materials to form a crystal matrix in the absorber.
Although the one-step co-evaporation process is of interest to CIGS manufacturing, the two-step process may be more manufacturable and poses unique challenges of its own. In the first step of the two-step process, a material is deposited upon a substrate. The material deposited on the substrate is referred to as the “precursor.” The precursor may comprise one or more of copper (Cu), indium (In), gallium (Ga), and/or selenium (Se) and/or sulfur (Se). Usually the precursor is a mixture of copper, indium, and gallium. In the second step of the two-stage process of CIGS manufacturing, selenium or sulfur is introduced into the precursor by a process known in the art as “selenization.” Selenization typically includes heating the precursor in a selenium-rich (or sulfur-rich) environment until the elements react to make a crystal matrix to form the chalcopyrite CIGS material that becomes known as the “absorber.” Common sources of selenium or sulfur in CIGS manufacturing include vaporizing powdered selenium or sulfur, hydrogen selenide, hydrogen sulfide, or organic compounds of selenium or sulfur with low evaporation points. This process has been accepted by researchers in solar cell manufacturing methods as an acceptable means of introducing selenium or sulfur into the absorber material; however, this technique also poses substantial risks and costs. Further, as some researchers may blend certain elements of the one-stage and two-stage process, these challenges may apply to the one-stage process as well.
Selenization is usually practiced by two methods. In one prior art method, selenium pellets are placed in a receptacle, or “boat,” in a chamber and then the selenium and precursor are heated to release a selenium-containing vapor which interacts with the precursor. In the other prior art method the treatment chamber is filled with selenium or sulfur vapor or with hydrogen selenide (H2Se) or hydrogen sulfide (H2S) gas. Sometimes a process will involve placing hydrogen (H2) gas in the treatment chamber while heating the Se or S pellets to form H2Se or H2S in situ. These two methods are essentially the only methods of selenizing photovoltaic precursors.
Due to the nature of the chemical reactions, an excess amount of Se or an over-pressure of Se is desirable during the selenization process. An excess of Se is typically necessary since the reaction of the Cu, In, Ga, and Se tends to “push” at least some of the Se out of the precursor at elevated temperature. Therefore, it is believed that, without excess Se present, any deposited Se will tend to evaporate out of the precursor matrix and not bind to the matrix as desired. Aspects of the present invention overcome this barrier by providing sufficient Se to minimize the escape of Se from, for example, the Cu—In—Ga matrix.
With regards to thermal co-evaporation, some prior art co-evaporation processes “hint” that selenization may be used to “fix” a film that might not be quite right stoichiometrically. That is, after co-evaporation, the precursor may lack sufficient Se whereby further Se addition is required to provide the desired stoichiometric quantity of Se. This further selenization is typically practiced by one of the methods discussed above.
Current CIGS manufacturing techniques also have serious health and environmental implications. As discussed below, various manufacturing techniques have been used to introduce selenium or sulfur into the absorber material matrix with varying success. Although some manufacturing methods are more reliable, the health or environmental concerns, especially in large-scale production volumes, make them undesirable for long-term use. More specifically, the use of the highly toxic hydrogen selenide and its derivatives is expensive because of needed safety precautions. While CIGS solar cells show great promise in solar cell manufacturing to reduce raw material costs, safe, reliable, and repeatable methods to introduce selenium or sulfur into the matrix are needed.
Prior art also suggests that CIGS solar cells produced by selenization processes have performance problems that may be unique to the manufacturing method. Recent studies by P. K. Johnson and A. E. Delahoy showed that solar cells produced by selenization had higher defect densities, “light-inhibited” degradation of cell efficiency of up to 97%, and a 13% reduction in Voc×FF over a 30 to 45 day period. In contrast, solar cells produced by thermal co-evaporation showed lower defect densities, lower cell efficiency reduction, and less than a 2% reduction in Voc×FF over a 30 to 45 day period. The key distinguishing feature of most selenization processes and thermal co-evaporation is that selenization usually uses a hydrogen-containing species, H2Se. Although some of the decreased product performance of selenized solar cells is due to encapsulation method of the module and migration of sodium from the soda lime substrate into the absorber matrix, a good portion of the discrepancies in cell performance have to do with manufacturing method. While the enhanced product performance factors make thermal co-evaporation more desirable, selenization process methods are more suited to manufacturing high efficiency cells on large area substrates.
Additionally, H2Se is incompatible with stainless steel and other metals that have the potential to replace soda lime glass as a substrate material. This distinction is increasingly important as solar cell manufacturers look to lower manufacturing costs while increasing the number of form factors available for “finished” solar cell devices. Thus, in addition to the safety and environmental concerns, a solar cell manufacturing method that comprises 1) the low hydrogen advantages of thermal co-evaporation on long term cell performance, 2) the manufacturing capability high efficiency solar cells on large area substrates, and 3) compatibility with stainless steel and other metals is also needed.